1. Field of the Invention
This invention relates to a physical quantity detection device for generating a physical quantity detection signal.
2. Description of the Prior Art
Generally, pressure sensors for vehicles operate with a single power supply of 5 V, and the output range is from 0.5 to 4.5 V. Outside this range within 0 to 5 V, there are error detection ranges from 0 to 0.3 V and from 4.7 to 5V as shown in FIG. 9. FIG. 9 is a graphical diagram illustrating an output range and error detection ranges in a prior art pressure sensor. If the pressure sensor generates its output at the error detection range, a diagnostic function provided to a control system for the pressure sensor or the like detects the error only by comparing the detected voltage with reference voltages, i.e., 0.3 V and 4.7 V.
FIGS. 6 and 7 show interconnections when the pressure sensor is used in a vehicle in the prior art pressure sensors. The difference between FIGS. 6 and 7 is that in FIG. 6, a pull-up resistor 108 is provided, on the other hand, in FIG. 7, a pull-down resistor 105 is provided. Connector assembly 101, which includes connectors 100a, 100b, and 100c, and connector assembly 102 provide interconnections between pressure sensor 100 and system control circuit 104. These interconnections between the pressure sensors 100 and the system control circuits 104 provide disconnection detection.
For example, in FIG. 7, the pressure sensor 100 is connected to the system control circuit 104 with a wire cable 103a for power supply, a wire cable 103b for outputting the detection signal, and a wire cable 103c for grounding, wherein the line connected to the output signal wire cable in the system control circuit 104 is connected to the ground through a pull-down resistor 105. Then, if the wire cable 103a or its connector 100a or the wire cable 103b or its connector 100b is disconnected, the detected voltage at the wire cable 103b becomes zero volts. If the ground line 103c or its connector 100c is disconnected, the detected voltage on the wire cable 103b becomes higher than 4.7 V due to voltage dividing with the internal resistance of the pressure sensor 100 and the pull-down resistor 105. A CPU 107 detects this voltage as an error signal through an A/D converter 106. Then, the CPU 107 judges that there is disconnection between the pressure sensor 100 and the system control circuit 104.
In the circuit structure shown in FIG. 6, the disconnection condition is detected in the similar manner.
FIG. 8 is a schematic circuit diagram of a prior art pressure sensor. This prior art pressure sensor includes resistors Ra, Rb, Rc, and Rd as strain gages which are formed in a diaphragm arranged at a middle of an Si chip. When a pressure on the diaphragm increases, the resistances of the resistors Ra and Rd decreases. On the other hand, resistances of the resistors Rb and Rc increase. Thus, these resistors Ra, Rb, Rc, and Rd form a Wheatstone bridge.
Resistors R1, R2, R3, R4, R5, R6, R71, R72, R81, R82, R9, R10, R11, and R12 other than the resistors Ra, Rb, Rc, and Rd comprise thin film resistors such as CrSi films of which temperature coefficients of resistance TCRs are almost zero.
The resistors R1 and R2 divide the supply voltage Vcc to generate a middle voltage at the junction point between these resistors which is used as a reference voltage for operational amplifiers OP10 and OP40.
The operational amplifier OP10 and the resistors R1, R2, and R5 form a constant current source for driving the Wheatstone bridge. This constant current source keeps the constant current supply irrespective of temperature variation because the temperature coefficient of resistance of the resistor R5 is almost zero.
The strain gages operate such that if they are driven with a constant current, the sensitivity in pressure is temperature-compensated. That is because the strain gages are formed of p type diffused resistors including impurity at a concentration of about 1020 cmxe2x88x923. This fact is well known. Moreover, the resistors R71, R72, R81, and R82 are used for zero point adjustment of the Wheatstone bridge by trimming the resistors R71, R72, R81, and R82 with laser. The resistor R6 is connected in parallel with the Wheatstone bridge for fine adjustment of temperature characteristic in sensitivity.
The operational amplifiers OP20 and OP30 are provided as voltage follower circuits supplied with the voltages at the junction points of the Wheatstone brides. More specifically, an output of the operational amplifier OP20 is connected to a transistor T1 which is connected to a transistor T2 with Darlington connection. The operational amplifier OP40 operates as an amplifier and an adder. The gain of the operational amplifier for the pressure signal is R12/R9. The inverting input of the operational amplifier OP40 is connected to the supply power VCC1 through the resistor R11, so that zero point of the sensor output VO1 is adjusted by trimming the resistor R11. The resistors R10, R3, and R4 are used for temperature compensation of the zero point by trimming the resistor R3 or the resistor R4. Here, the resistor R10 has a larger resistance than the resistors R3 and R4.
This circuit operates with reference to the above-mentioned reference voltage generated by dividing the supply voltage VCC1. Thus, if the supply voltage VCC1 varies within an allowable range, the output voltage VO1 varies in proportion to the variation of the supply voltage VCC1. More specifically, the supply voltage VCC1 is commonly used between the A/D converter 106 in the system control circuit 104 and the reference voltage generation portion in the pressure sensor. This suppresses the error in the pressure detection signal (VO1) with respect to variation in supply voltages.
FIG. 9 is a graphical drawing of voltage ranges for error detection in the prior art pressure sensor. FIG. 10 is an interconnection diagram of the prior art pressure sensor for a vehicle. The pressure sensor 110 is connected to the system control circuit 111 through the cables and connectors because the pressure sensor 110 is located remote from the system control circuit 111. In this interconnection, if a contact resistance in a connector increases, the output voltage may become an intermediate voltage outside the error detection range shown in FIG. 9.
This condition could not be detected with the pull-down resistor or the pull-up resistor.
The aim of the present invention is to provide a superior physical quantity detection device.
According to the present invention, a first aspect of the present invention provides a physical quantity detection device supplied with a supply voltage from a system control circuit having a function for varying said supply voltage, comprising: a sensor circuit for generating a detection signal corresponding to a physical quantity to be measured; and an output circuit for outputting said detection signal when said supply voltage is within a predetermined voltage range and for generating and outputting a predetermined voltage which is irrespective of said physical quantity when said supply voltage is outside said predetermined voltage range.
According to the present invention, a second aspect of the present invention provides a physical quantity detection device on the basis of the first aspect, wherein said sensor circuit comprises: a bridge circuit for generating said detection signal corresponding to said physical quantity; and wherein said outputting circuit comprising: a voltage follower circuit coupled to said bridge circuit; and control means for controlling an output of said voltage follower circuit such that said voltage follower circuit outputs said detection signal when said supply voltage is within said predetermined voltage range, and said voltage follower circuit generates said predetermined voltage which is irrespective of said detection signal when said supply voltage is outside said predetermined voltage range.
According to the present invention, a third aspect of the present invention provides a physical quantity detection device on the basis of the second aspect, wherein said control means comprises a first transistor turning on and off on the basis of said supply voltage, wherein said first transistor turns off when said supply voltage is within said predetermined voltage range and turns on to make the output of said voltage follower circuit go a low voltage level when said supply voltage is outside said predetermined voltage range.
According to the present invention, a fourth aspect of the present invention provides a physical quantity detection device on the basis of the second aspect, further comprising: an adder including an operational amplifier of which inverting input is supplied with said detection signal and of which non-inverting input is supplied with a reference voltage generated by voltage-dividing said supply voltage and a first resistor connected between said inverting input and an output of said operational amplifier; and current changing means for changing a current flowing through said first resistor when said supply voltage is outside said predetermined voltage range to make said operational amplifier output a predetermined voltage signal.
According to the present invention, a fifth aspect of the present invention provides a physical quantity detection device on the basis of the fourth aspect, wherein said current changing means comprises a transistor and a second resistor connected between said inverting input of said operational amplifier and said transistor, wherein said transistor turns on when said supply voltage is outside said predetermined voltage range to flow a predetermined current through said second resistor.
According to the present invention, a sixth aspect of the present invention provides a physical quantity detection device on the basis of the first aspect, further comprising first to third cables for connecting said sensor circuit to said system control circuit, said first cable supplying said supply voltage from said system control circuit to said sensor circuit, said second cable supplying said detection signal to said system control circuit, and said third cable connecting a ground of said physical quantity detection device to a ground of said system control circuit.
According to the present invention, a seventh aspect of the present invention provides a physical quantity detection device supplied with a supply voltage from a system control circuit having a function for varying said supply voltage comprising: a sensor circuit for generating a detection signal corresponding to a physical quantity to be measured; and an output circuit for outputting said detection signal when said supply voltage is within a first predetermined voltage range and for generating and outputting a predetermined voltage which is irrespective of said physical quantity when said supply voltage is within a second predetermined voltage range.
According to the present invention, an eighth aspect of the present invention provides a physical quantity detection device, comprising: a sensor circuit for generating a detection signal in accordance with a physical quantity; and an outputting circuit for outputting said detection signal when said supply voltage is within a predetermined voltage range and for generating and outputting a predetermined voltage signal when said supply voltage is outside said predetermined voltage range.
According to the present invention, a ninth aspect of the present invention provides a physical quantity detection device on the basis of the seventh aspect, wherein said sensor circuit is fixedly connected to said outputting circuit.
According to the present invention, a tenth aspect of the present invention provides a physical quantity detection device on the basis of the seventh aspect, further comprising: a system control circuit for generating said supply voltage within said predetermined voltage range in a first mode and outside said predetermined voltage range in a second mode; a first cable including connectors for supplying said supply voltage from said system control circuit to said sensor circuit and said outputting circuit; a second cable including connectors for supplying said detection signal and said predetermined voltage signal to said system control circuit; and a third cable including connectors for connecting a ground of said bridge circuit and said outputting circuit to a ground of said system control circuit.
According to the present invention, an eleventh aspect of the present invention provides a physical quantity detection device on the basis of the ninth aspect, further comprising: a judging circuit for judging whether said voltage signal is within an allowable voltage range and outputting a judging result in said second mode to judge conditions of said connectors of said first to third cables.
According to the present invention, a twelfth aspect of the present invention provides a physical quantity detection device on the basis of the ninth aspect, wherein said system control circuit comprises: a first power supply for generating said supply voltage; a second power supply for generating another supply voltage and for generating a voltage data of said another supply voltage, said first power supply generates said supply voltage within said predetermined voltage range in accordance with said voltage data to equalize said supply voltage to said another supply voltage in said first mode, said physical quantity detection device further comprising: an A/D converter supplied with said another supply voltage for converting said detection signal into a digital detection signal, wherein voltage characteristic of said bridge circuit and a voltage characteristic of said A/D converter is compensated by equalizing said supply voltage to said another supply voltage in said first mode.